Retaining wall backfill idealization: The standard design assumption for retained earth behind a retaining wall is that the backfill is __________.

Introduction / Context:

Retaining wall design relies on earth pressure theories (e.g., Rankine or Coulomb), which are derived under idealized conditions for the retained soil. To apply these theories safely and consistently, the designer adopts standard backfill assumptions that simplify earth pressure to a function of unit weight, friction angle, and geometry, avoiding complicating factors like cohesion or hydrostatic pressure where possible.

Given Data / Assumptions:

• Wall retains an idealized backfill.
• Use of classical active/passive earth pressure theory.
• Backfill behavior should be predictable and drain well.

Concept / Approach:

Classical design assumes the retained soil is granular (cohesionless), effectively dry (i.e., free from significant pore water pressures), homogeneous, and with a plane surface. These conditions maximize the applicability of Rankine/Coulomb formulations and reduce uncertainties due to cohesion and water pressure buildup.

Step-by-Step Solution:

Verification / Alternative check:

Practical designs may include drainage layers and weepholes to maintain ‘‘dry’’ conditions. If cohesion or water pressure exists, modified parameters or additional checks (e.g., effective stress with water table) are required.

Why Other Options Are Wrong:

• Any single condition alone (dry, cohesionless, granular) does not fully capture the standard set of simplifying assumptions; the complete and commonly taught assumption set is all of them together.

Common Pitfalls:

• Ignoring drainage: if water accumulates, effective lateral pressure increases markedly.
• Assuming cohesive backfills without adjusting theory; cohesion alters active/passive states and may be unreliable long term.

Final Answer:

all the above